The present invention relates to a method of manufacturing a multilayer wiring board and a multilayer wiring board obtained thereby and, more specifically, a method of manufacturing a multilayer wiring board in which via holes are filled with conductive paste containing metal powder.
In recent years, in association with a trend toward reduction of thickness and length of products in the field of electronics, the similar needs are increasing also for a printed wiring board. Multi-layering is inevitable in miniaturization of the printed wiring board and, in order to achieve the multi-layering, reduction of thickness is a technical challenge.
As a method of manufacturing the multilayer wiring board of the related art, there is a method including (a) providing a via hole on a core board 10 having copper layer layers 11 on both surfaces of an insulated material, applying a through hole plating 12 on a wall surface of the via hole and then printing, filling and hardening via-fill paste 13 therein, then (b) polishing projecting portions of the filled conductive paste 13, and then (c) applying cap plating 14 on the both surfaces, as shown in FIG. 4 for example. There is also a method of filling a via hole by using a resin of a resin layer 15 of a core board 10 by disposing copper layers (CCL) 16 having a layer 15 applied with a resin on both sides of the core board 10, with the resin layers 15 facing the core board 10, as shown in FIG. 5(a), and pressing the same as shown in (b).
However, the former has a probability of extension of the board after having polished the paste and hence the reduction of the thickness of the film is limited, while the cap plating results in increase in thickness of the copper layers on the surfaces which causes a difficulty of fine patterning. In contrast, the latter has problems of an inability to provide a via hole on a via hole (stacked via), insufficient smoothness of the surface of the board, and generation of cracks at the time of a long-term reliability test.
In cope with these problems, a method of obtaining an internal layer conductivity only with the conductive paste without performing the through hole plating or the cap plating as described above, more specifically, a press method using paste which develops conductivity by contact between conductive powder and conductive powder (hereinafter, referred to as powder contact paste) is employed. However, utilization of such paste causes a problem of generation of roughness because the board immediately above the paste is swelled under normal pressing conditions, and also causes a problem of insufficient long-term reliability because the change rate of conductivity over time is high.
In contrast, the fact that the conductivity and the long-term stability can be improved by using paste containing metal powder which is alloyed by being heated (hereinafter referred to as “alloying paste”) and forming alloy between the conductive powders and between the powder and the copper layer is disclosed (Japanese Patent No. 4191678).
In contrast, pre-preg is generally used as a material of the printed board (JP-A-7-176846). The term “pre-preg” means an adhesive sheet obtained by processing thermosetting resin into a state of B-stage, and is normally a glass fiber cloth or the like as a reinforcing material impregnated with a thermosetting resin. When manufacturing the board using the pre-preg described above, a via hole is formed through the pre-preg by laser beam machining as in the related art, conductive paste is filled therein, copper layers or patterned boards are arranged on upper and lower surfaces thereof, and pressing is performed.
However, when forming a through hole with the alloying paste described above, it is necessary to slow down a curing speed of the paste resin component because alloying is hindered if the paste resin component is completely cured before a low melting metal is melted. However, a general pre-preg has problems in that if the curing speed of the paste resin component is slowed down, the conductive paste may spread at the time of pressing, and the resin component of the pre-preg and the paste may be mixed, thereby failing to obtain desired characteristics.